FIG. 1 shows a typical bag made of a polymer film by automatic packaging machines or bag-making machines, where 1 is the top seal, 2 is the bottom seal and 3 is the back seal. All of these seals are made by heat sealing one end of the film with the other end of the film. There are two kinds of heat sealing methods shown in FIGS. 2 and 3. The heat sealing method shown in FIG. 2 is called a "fin-seal"; in this method, one inner side of the bag is heat sealed with the other inner side of the bag. On the other hand, the heat sealing method in FIG. 3 is called a "lap-seal"; in a "lap-seal", an inner side of the bag is heat sealed with an outer side of the bag.
The three-layered polypropylene film of this invention is particularly suitable for forming bags with a "lap-seal". There are many two-layered polypropylene films used in the market, consisting of a biaxially oriented polypropylene film (hereinafter referred to as PP-BO) and a laminated ethylenepropylene copolymer (hereinafter referred to as EPC) layer. However, these two-layered films cannot be used to form a lap-seal, because the sealability of the PP-BO is so poor that good heat sealing cannot be achieved between the PP-BO and the EPC sides. Three-layered polypropylene films, consisting of a core layer of PP-BO sandwiched by two EPC layers, are also well-known in the patent literature, such as Japanese Pat. Nos. 46-31478, 47-39225, 47-45951, and 48-4189, U.S. Pat. Nos. 3,620,825 and 3,671,383, and British Pat. Nos. 1,055,803, 1,172,643 and 1,440,317. Due to EPC layers on both sides of the films, these three-layered films can be used for lap-seal. However, these films have a quality defect of either (1) poor heat sealability, or (2) poor resistance against "hot-tack", "blocking" and "scratches".
In these three-layered films of the prior art, a film with good heat sealability has poor resistance, while a film with good resistance has poor heat sealability. For example, the three-layered films made by the methods in Japanese Pat. No. 46-31478 or U.S. Pat. No. 3,671,383 are good in heat sealability but poor in resistance against "hot-tack" etc. On the other hand, the three-layered films made by the methods in Japanese Pat. No. 47-39225 or British Pat. No. 1,172,643 are good in resistance but poor in heat sealability. Good heat sealability, as used herein, means that the film can be heat sealed over a wide range of temperatures, and that the heat sealed parts have a sufficient heat seal strength, e.g., more than 200 g/cm.
"Hot-tack" is a phenomenon wherein the hot heat sealing bar sticks to the film when the film is heat sealed in the automatic packaging machines or bag-making machines. It is easily understood that smooth packaging operations cannot be made if "hot-tack" occurs. The film with heat sealing layers on both sides is particularly vulnerable to "hot-tack", because the hot sealing bar directly contacts the heat sealing layers. It is readily apparent that "hot-tack" is an undesirable property in packaging applications.
"Blocking" is a phenomenon wherein the film surfaces stick to other film surfaces. This often happens during storage of the films, bags or packages. A film with a "blocking" propensity is inconvenient in handling because it takes time to separate the blocked films, bags or packages. Hence, "blocking" is also an undesirable property in packaging applications.
"Scratches" are fine lines or marks resulting from mechanical friction between films or film and other materials, i.e., "scratches" take place on the film surface when a film is rubbed with another film or material. Good appearance, or clarity, of the film is damaged by "scratches", so that good resistance against "scratches" is required for packaging films.